Dual Magnetic Tunnel Junction Devices for Magnetic Random Access Memory (MRAM)

ABSTRACT

A dual magnetic tunnel junction (DMTJ) is disclosed with a PL 1 /TB 1 /free layer/TB 2 /PL 2  configuration wherein a first tunnel barrier (TB 1 ) has a substantially lower resistance x area (RA 1 ) product than RA 2  for an overlying second tunnel barrier (TB 2 ) to provide an acceptable magnetoresistive ratio (DRR). Moreover, first and second pinned layers, PL 1  and PL 2 , respectively, have magnetizations that are aligned antiparallel to enable a lower critical switching current that when in a parallel alignment. The condition RA 1 &lt;RA 2  is achieved with one or more of a smaller thickness and a lower oxidation state for TB 1  compared with TB 2 , with conductive (metal) pathways formed in a metal oxide or metal oxynitride matrix for TB 1 , or with a TB 1  containing a dopant to create conducting states in the TB 1  band gap. Alternatively, TB 1  may be replaced with a metallic spacer to improve conductivity between PL 1  and the FL.

PRIORITY DATA

The present application is a divisional application and claims thebenefit of U.S. patent application Ser. No. 16/056,791 filed Aug. 7,2018, herein incorporated by reference in its entirety.

RELATED PATENT APPLICATIONS

This application is related to the following: Docket # HT17-014, Ser.No. 15/841,479, filing date 12/14/17; and Docket # HT17-034, Ser. No.15/728,818, filing date 10/10/17, which are assigned to a commonassignee and herein incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a dual magnetic tunnel junction (DMTJ)comprised of a free layer that interfaces with a lower tunnel barrier(TB1) layer and an upper tunnel barrier (TB2) layer, and wherein the TB1layer has a resistance x area (RA) product substantially less than theTB2 layer, and spin polarizer layers adjoining the TB1 and TB2 layersare initialized antiparallel to each other to significantly reduce thecritical current for switching the free layer.

BACKGROUND

Perpendicularly magnetized MTJs (p-MTJs) are a major emerging technologyfor use as embedded magnetic random access memory (MRAM) applications,and standalone MRAM applications. P-MTJ MRAM technology usingspin-torque (STT-MRAM) for writing of memory bits was described by C.Slonczewski in “Current driven excitation of magnetic multilayers”, J.Magn. Magn. Mater. V 159, L1-L7 (1996), and is highly competitive withexisting semiconductor memory technologies such as SRAM, DRAM, andflash.

Reducing the critical switching current density for p-MTJs is a keychallenge for integrating MRAM and STT-MRAM into existing complementarymetal oxide semiconductor (CMOS) technologies. As the write current isreduced, smaller transistors may be used for each bit cell therebypotentially enabling higher density memory arrays and lower productioncost. One of the strategies explored in the past for minimizing thecritical current (i_(c)) for switching the free layer in a p-MTJ is adual spin filter structure also referred to as a DMTJ. A typical DMTJhas a PL1/TB1/FL/TB2/PL2 configuration wherein PL1 and PL2 are first andsecond pinned layers, that adjoin first and second tunnel barrier layersTB1 and TB2, respectively, and create a spin torque effect on the freelayer (FL) when a current is passed through the DMTJ in a perpendicularto plane direction. Preferably, each of PL1, PL2, and the FL has amagnetization aligned in a perpendicular to plane (vertical) direction.When PL1 and PL2 are initialized anti-parallel to each other, there ispotentially a two-fold increase in the spin torque on the FL comparedwith a MTJ having a single spin polarizer in a PL/TB/FL configuration,for example. As a result, there is improved spin torque transferefficiency and a reduction in (i_(c)).

In the aforementioned DMTJ, the PL1/TB1/FL stack may be considered as afirst p-MTJ substructure while the FL/TB2/PL2 stack may be considered asa second p-MTJ substructure. In each p-MTJ substructure, the FL is freeto rotate to a direction that is parallel (P state) or antiparallel (APstate) with respect to PL1 and PL2. It is important for the netmagnetoresistive ratio (DRR) to be a large value, preferably higher than1, as DRR is directly related to the read margin.

The magnetic performance for a DMTJ with two p-MTJ substructures isrelated not only to DRR and i_(c), but also to the difference (RA₂−RA₁).In the prior art, one or two of these parameters are addressed with anew design, but there is a need to optimize all three simultaneously,and to reduce i_(c) lower than that achieved with a single p-MTJ cell.Therefore, an improved DMTJ structure is needed where i_(c) is minimizedwithout a substantial sacrifice in DRR, and without increasing RA to anunacceptably high level that would lead to a decreased lifetime for oneor both tunnel barrier layers.

SUMMARY

One objective of the present disclosure is to provide a DMTJ cell designthat reduces the critical current (i_(c)) for switching the free layerto less than the i_(c) necessary to toggle a single p-MTJ cell whileenabling acceptable DRR and RA for advanced MRAM and STT-MRAM devices.

A second objective is to provide a method of initializing the pinnedlayers in a DMTJ that is compatible with the DMTJ cell design of thefirst objective.

According to a first embodiment of the present disclosure, a preferredDMTJ cell also known as a dual spin filter (DSF) comprises a free layer(FL) sandwiched between a lower first tunnel barrier layer (TB1) and anupper second tunnel barrier layer (TB2). There is also a first pinnedlayer (PL1) adjoining a bottom surface of TB1, and a second pinned layer(PL2) contacting a top surface of TB2. Thus, a first p-MTJ substructureof the DMTJ has a PL1/TB1/FL stack of layers while the second p-MTJsubstructure has a FL/TB2/PL2 stack. All magnetic layers (PL1, PL2, FL)have a magnetization in a perpendicular to plane (vertical) directionthat is orthogonal to a top surface of a substrate on which the DMTJ isformed. Moreover, the DMTJ is initialized in a so-called working statesuch that PL1 magnetization is antiparallel to that for PL2 to enable alower is than when PL1 and PL2 magnetizations are parallel (non-workingstate), or compared with a single p-MTJ that switches back and forthbetween P and AP states. In the DMTJ working state, the first p-MTJ hasa P state while the second p-MTJ has an AP state to give a P/APconfiguration for the DMTJ, or the first p-MTJ has an AP state while thesecond p-MTJ has a P state to give an AP/P configuration.

Another key feature of the DMTJ working state is that the RA of TB1hereafter referred to as RA₁ is substantially less than the RA of TB2hereafter referred to as RA₂ so that the net DRR is maximized comparedwith a DMTJ where RA₁=RA₂ and where intrinsic magnetoresistance valuesfor TB1 and TB2 are equivalent which results in a net DRR=0.

According to some embodiments, both of TB1 and TB2 are metal oxidelayers. RA₂ is substantially larger than RA₁ because TB2 has one or bothof a greater thickness than TB1, and a higher oxidation state. Forexample, TB2 may have a stoichiometric oxidation state where essentiallyall sites in the metal oxide lattice that are not occupied with metalatoms are filled with oxygen atoms, and TB1 may be substantiallyunderoxidized with a plurality of sites in the metal oxide lattice thatare not occupied by oxygen atoms.

In other embodiments, TB2 may be a metal oxide layer with astoichiometric oxidation state while TB1 is a metal oxide matrix withconductive channels formed therein to lower resistivity and RA₁ in TB1.In alternative embodiments, the metal oxide matrix in TB1 may bereplaced by a metal oxynitride matrix or metal nitride matrix havingconductive channels therein. In yet another embodiment, TB1 is a metaloxide layer that is doped with one of N, S, Se, P, C, Te, As, Sb, or Sithat creates conductivity states in the band gap of the TB1 layer.

The present disclosure also encompasses a method of forming a TB1 layerwhere RA₁ is substantially less than RA₂ of an overlying TB2 layeraccording to a DMTJ embodiment previously described. Methods are alsoprovided for initializing a DMTJ such that PL1 magnetization is oppositeto PL2 magnetization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are cross-sectional views of a DMTJ wherein the two pinnedlayers (PL1 and PL2) have magnetic moments aligned parallel andantiparallel to each other, respectively, and all magnetic layersincluding the free layer (FL) have perpendicular-to-planemagnetizations.

FIG. 1C is a cross-sectional view of a DMTJ wherein the two pinnedlayers have SyAP configurations, and the AP1 layer in PL1 is alignedantiparallel to the AP1 layer in PL2 according to an embodiment of thepresent disclosure.

FIG. 2 is a cross-sectional view of a single p-MTJ in the prior art.

FIG. 3A depicts the DMTJ configuration in FIG. 1A where PL1 and PL2magnetizations are parallel, and a current is applied to switch a P/Pstate to an AP/AP state, or a reverse current switches the AP/AP stateto a P/P state.

FIG. 3B depicts the DMTJ configuration in FIG. 1B where PL1 and PL2magnetizations are antiparallel, and a current switches a P/AP state toan AP/P state, or a reverse current switches an AP/P state to a P/APstate.

FIG. 4 is a cross-sectional view wherein the TB1 layer in FIG. 1B has alower RA than the TB2 layer because of conductive channels formed withina TB1 insulator matrix according to an embodiment of the presentdisclosure.

FIGS. 5-7 are cross-sectional views depicting various methods of formingconductive M2 channels in a metal oxide (MO_(x)) matrix according toembodiments of the present disclosure.

FIG. 8 is a cross-sectional view depicting an intermediate step informing a DMTJ wherein a lower tunnel barrier layer has a substantiallylower RA₁ product than RA₂ in an upper tunnel barrier layer.

FIG. 9 is a cross-sectional view wherein the TB1 layer in FIG. 1B has alower RA than the TB2 layer because the TB1 layer is doped to createconductivity states in the TB1 band gap according to an embodiment ofthe present disclosure.

FIG. 10 is a cross-sectional view wherein the TB1 layer in FIG. 1B has alower RA than the TB2 layer because the TB1 layer is comprised of aconductive metal layer according to an embodiment of the presentdisclosure.

FIGS. 11-12 are cross-sectional views depicting an initializationsequence comprised of two applied fields to provide the DMTJconfiguration in FIG. 1B.

FIG. 13 is a scheme showing an initialization sequence involving anapplied voltage to provide the DMTJ configuration in FIG. 1B.

FIG. 14 is a plot of resistance as a function of voltage for the P/APand AP/P states in FIG. 3B, and for the P/P and AP/AP states in FIG. 3A.

FIG. 15 is a table listing the free energy of oxide formation forvarious elements.

DETAILED DESCRIPTION

The present disclosure is a DMTJ that is configured to enable a lowercritical switching current density than realized in a single spin filterwhile providing acceptable DRR and (RA₁+RA₂) for the device, andfeatures a free layer (FL) formed between a lower tunnel barrier layer(TB1) and an upper tunnel barrier (TB2) layer wherein TB1 has a lower RAproduct than that of TB2. Moreover, a first pinned layer (PL1) thatcontacts a bottom surface of TB1 has a vertical magnetization that isaligned anti-parallel to a vertical magnetization of a second pinnedlayer (PL2) contacting a top surface of TB2. The DMTJ may beincorporated in a MRAM, STT-MRAM, or in another spintronic device suchas a spin torque oscillator (STO), sensor, or biosensor. The term“oxidation state” refers to the oxygen content in TB1 and TB2 layerscomprised of a metal oxide or metal oxynitride. A top surface for alayer is defined as a surface facing away from the substrate while abottom surface faces the substrate. An interface is a boundary regioncomprised of a bottom surface of one layer and an adjoining top surfaceof a second layer. A thickness of each DMTJ layer is in a z-axisdirection while the planes of the layers are laid out in the x-axis andy-axis directions.

In related application Ser. No. 15/841,479, we disclosed various methodsof minimizing RA in a metal oxide Hk enhancing layer in a p-MTJincluding reducing outer portions of the metal oxide layer, or formingconductive pathways therein. Similarly, in related application Ser. No.15/728,818, we disclosed a doped metal oxide Hk enhancing layer wherethe dopant is one of N, S, Se, P, C, Te, As, Sb, or Bi to fill vacantsites in a metal oxide lattice thereby lowering RA.

Here we disclose a DMTJ wherein the TB1 layer is designed with astructure that provides a RA₁ product that is lower than a RA₂ productin the TB2 layer in order to decrease the critical current densityrequired to switch the FL magnetization while providing acceptable DRR.Accordingly, one or more of the schemes disclosed in related patentapplications to increase conductivity in a Hk enhancing layer may alsobe applied to a tunnel barrier layer that has a metal oxide or metaloxynitride composition.

Referring to FIG. 1A, DMTJ 1 a is shown in which an optional seed layer11, PL1 12, TB1 13, FL 14, TB2 15, PL2 16, and hard mask or cappinglayer 17 are sequentially formed on a substrate 10. The DMTJ is depictedwith a P/P magnetic state wherein a first p-MTJ substructure 8 has PL1magnetization 12 m aligned parallel to FL magnetization 14 m (P state),and a second p-MTJ substructure 9 has PL2 magnetization 16 m alignedparallel to the FL magnetization (P state).

FIG. 3A shows another representation of the two parallel (P) states inFIG. 1A. When a sufficiently large write (switching) current 11 isapplied, FL magnetization flips from 14 m to 14 a thereby establishingan AP/AP state for DMTJ 1 a where FL magnetization 14 a is anti-parallelto both PL1 magnetization 12 m and PL2 magnetization 16 m. It should beunderstood that since magnetizations 12 m and 16 m are in the samedirection, the spin torque effect on the FL generated by current (l₁ orl₂) passing through PL1 effectively cancels the spin torque effect ofPL2 on the FL magnetization. As a result, a higher switching current isrequired for DMTJ 1 a compared with a single p-MTJ 2 shown in FIG. 2.The same outcome occurs when write current 12 is applied to switch DMTJ1 a from an AP/AP state to a P/P state which means the magneticorientations of PL1, PL2, and the FL in FIG. 3A are unfavorable in termsof the desired outcome of a reduced switching current compared with asingle p-MTJ structure. Hereinafter, the magnetic orientations of thePL1, PL2, and FL layers in FIG. 3A are referred to as a non-workingstate with regards to the objective of designing a DMTJ having a loweri_(c) than p-MTJ 2.

FIG. 1B illustrates an alternative configuration for the magnetizationsin PL1, PL2, and the FL of DMTJ 1 b. In particular, PL1 magnetization 12m is antiparallel to PL2 magnetization 16 a while FL magnetization 14 mis aligned parallel to 12 m to give a P/AP state for the dual spinfilter structure. Otherwise, all layers in the p-MTJ substructures 8, 9are retained from FIG. 1A.

In FIG. 3B, another representation of the P/AP state in FIG. 1B isdepicted. Here, write current 13 is applied to switch FL magnetization14 m to 14 a thereby establishing an AP/P state for DMTJ 1 b where FLmagnetization is now antiparallel to PL1 magnetization 12 m but parallelto PL2 magnetization 16 a in the absence of an external magnetic field.As a result, a lower switching current is required for the DMTJ comparedwith a single p-MTJ since the spin torque effect generated by currentpassing through PL1 is added to the spin torque effect on the FLmagnetization from PL2 because magnetizations 12 m, 16 a areanti-parallel. Thus, the magnetic orientations of PL1, PL2, and the FLin FIG. 3B are favorable to reduce the critical switching currentrelative to a single p-MTJ, and compared with DMTJ 1 a in FIG. 3A. DMTJ1 b in FIG. 1B is hereinafter referred to as a working state for thepurpose of achieving the objectives of the present disclosure. Note thatthe same desirable outcome is realized by applying write current 14 toswitch FL magnetization 14 a to 14 m in FIG. 3B and change the DMTJ froman AP/P state to a P/AP state.

Referring to FIG. 2, a single spin filter (p-MTJ 2) is depicted whereinseed layer 11, pinned layer 3, tunnel barrier 4, free layer 5, optionalHk enhancing layer 6, and hard mask 17 are sequentially formed onsubstrate 10. The Hk enhancing layer is typically a MgO layer that isadvantageously used to form a second metal oxide interface with the FLthereby enhancing PMA and thermal stability. A critical current (notshown) is applied to switch the p-MTJ from a P state where FLmagnetization 5 m and PL magnetization 3 m are parallel to an AP statewhere 5 a and 3 m are antiparallel, or from an AP state to a P state.

In FIG. 1A, where PL1 and PL2 are aligned parallel, DMTJ 1 a may switchfrom a P/P state where FL magnetization is parallel to PL1 and PL2magnetizations to an AP/AP state where FL magnetization is antiparallelto PL1 and PL2 magnetizations. Let us consider the DRR values in DMTJ 1a as DRR₁=(R_(AP1)−R_(P1))/R_(P1) in p-MTJ substructure 8 andDRR₂=(R_(AP2)−R_(P2))/R_(P2) in p-MTJ substructure 9. In the magneticstate of FIG. 1A, we see that the resistance of the stack in the AP/APstate is R_(AP1)+R_(AP2), and in the P/P state is R_(P1)+R_(P2), and so,net DRR=(R_(AP1)+R_(AP2)−R_(P1)−R_(P2))/(R_(P1)+R_(P2)), which can thenbe simplified to (DRR₁*R_(P1)+DRR₂*R_(P2))/(R_(P1)+R_(P2)). Consideringa simple case where R_(P1)=R_(P2), we see that net DRR=DRR₁+DRR₂.

In FIG. 1B, where FL magnetization is parallel to one of PL1 and PL2 andantiparallel to the other, toggling the FL switches DMTJ 1 b back andforth between P/AP and AP/P states and the net DRR, using a similarapproach as above is represented by the equation netDRR=(R_(AP2)+R_(P1)−R_(P2)−R_(AP1))/(R_(P1)+R_(P2))=DRR₂*R_(P2)−DRR₁*R_(P1))/(R_(P1)+R_(P2)).Assuming, as in the previous case, that R_(P1)=R_(P2), net DRR is then(DRR₂−DRR₁)/2.

We find that when toggling the free layer from magnetization 14 m to 14a, or vice versa, the net DRR is 0 when TB1 and TB2 have the same RA,and equivalent intrinsic magnetoresistance. Therefore, the working stateDMTJ configuration only has acceptable DRR when RA₁ is substantiallydifferent from RA₂, or if there is a considerable difference inintrinsic magnetoresistance for TB1 and TB2.

It should be understood that the benefit of a lower switching currentdensity provided by DMTJ 1 b compared with p-MTJ 2 is not dependent onthe composition of the DMTJ layers 11-17. However, optimum performanceis achieved when RA₁ of TB1 13 is less than RA₂ of TB2 15. As thedifference (RA₂−RA₁) increases, the net DRR for DMTJ also increases.According to one embodiment, the condition RA₁<RA₂ is realized by one orboth of a smaller thickness for TB1 than TB2, and a lower oxidationstate for TB1 compared with TB2. Because the roughness (non-uniformity)of a DMTJ layer generally increases with increasing distance fromsubstrate 10, and a thin metal oxide layer is preferably grown(deposited) on a more uniform surface to prevent pinholes, TB1 ispreferably deposited before FL 14 and TB2. The present disclosure alsoencompasses various TB1 compositions for reducing RA₁ that are describedin later sections.

Seed layer 11 is formed on substrate 10 that may comprise a bottomelectrode and a substructure (not shown) including a bit line (or sourceline), and a transistor that are electrically connected to the BEthrough vias. The seed layer serves to induce or enhance perpendicularmagnetic anisotropy (PMA) in the overlying PL1 layer 12 and ispreferably comprised of one or more of NiCr, Ta, Ru, Ti, TaN, Cu, Mg, orother materials typically employed to promote a smooth and uniform grainstructure in overlying layers.

PL1 layer 12 may be a single ferromagnetic (FM) layer that is one orboth of Co and Fe, or an alloy thereof with one or both of Ni and B, ormay be a laminated stack with inherent PMA such as (Co/Ni)_(n),(CoFe/Ni)_(n), (Co/NiFe)_(n), (Co/Pt)_(n), (Co/Pd)_(n), or the likewhere n is the lamination number. In other embodiments, anantiferromagnetic (AFM) pinning layer (not shown) may be providedbetween the optional seed layer and reference layer to pin the PL1magnetization.

In yet another embodiment depicted in FIG. 1C, DMTJ 1 c is depicted andretains all of the DMTJ layers in FIG. 1B except PL1 12 may have asynthetic anti-parallel (SyAP) configuration represented by AP2/Ru/AP1where an anti-ferromagnetic coupling (AFC) layer 12-3 made of Ru, Rh, orIr, for example, is sandwiched between the AP2 FM layer 12-2 withmagnetization 12 a 2 and the AP1 FM layer 12-1 having magnetization 12 m1. The AP2 layer, which is also referred to as the outer pinned layer isformed on the seed layer while AP1 is the inner pinned layer andcontacts TB1 13. The AP1 and AP2 layers may be comprised of CoFe, CoFeB,Co, or a combination thereof, or each may be a laminated stack withinherent PMA such as (Co/Ni)_(n), (CoFe/Ni)_(n), (Co/NiFe)_(n),(Co/Pt)_(n), (Co/Pd)_(n), or the like where n is the lamination number.Furthermore, a transitional layer such as CoFeB or Co may be insertedbetween the uppermost layer in the laminated stack and TB1. Antiparallelorientation of AP1 and AP2 layers becomes the lowest energy state whenthe AFC layer has an appropriate thickness, which is about 4 Angstromswhen the AFC layer is Ru. Thus, the stability of the SyAP structure(orientations of 12 m 1 and 12 a 2) depends on the magnitude of theexchange interaction from AFC coupling, and on the anisotropy energy(perpendicular magnetic anisotropy or PMA) in the AP1 and AP2 layers.

According to the present disclosure, a key feature is that AP1magnetization 12 m 1 is antiparallel to the magnetization in PL2 16.When PL2 has a SyAP configuration, magnetization 16 a 1 in AP1 layer16-1 is preferably antiparallel to AP1 magnetization 12 m 1. Twodifferent initialization methods are described in a later section.Magnetization 16 m 2 in AP2 layer 16-2 is antiparallel to magnetization16 a 1 because of AF coupling through intermediate layer 16-3 that maybe Ru, Rh, or Os, for example.

In either embodiment (FIG. 1B or FIG. 1C), each of TB1 13 and TB2 15 ispreferably a metal oxide or metal oxynitride wherein the metal isselected from one or more of Mg, Ti, Al, Zn, Zr, Hf, and Ta. Moreover,one or both of TB1 and TB2 may be a lamination of one or more of theaforementioned metal oxides or metal oxynitrides. According to apreferred embodiment, TB2 has a stoichiometric oxidation state whereinessentially all metal atoms are completely oxidized with no vacant sitesin the metal oxide lattice in order to enhance the RA₂ product.Meanwhile, TB1 has a non-stoichiometric oxidation state, and preferablyhas substantially higher conductivity than TB2 so that RA₁ is minimizedcompared with RA₂. As previously mentioned, the difference (RA₂−RA₁) isdesirably increased when thickness t2 of TB2 is greater than thicknesst1 of TB1.

TB1 13 and TB2 15 are preferably fabricated with a radio frequency (RF)physical vapor deposition (PVD) process that is typically employed todeposit insulator films. For example, a MgO target may be RF sputterdeposited to yield one or both of TB1 and TB2. In some embodiments, a Mglayer (not shown) is deposited on PL1 12 with a RF PVD method. Then, theMg layer may be oxidized with a natural oxidation (NOX) process wherethe Mg layer is exposed to a flow of oxygen for a certain period oftime, or is oxidized with a conventional radical oxidation (ROX)process. Thereafter, an optional second Mg layer is deposited by a RFPVD method. During subsequent processes including one or more annealsteps, the second Mg layer becomes oxidized so that the MgO/Mgintermediate stack forms a MgO tunnel barrier layer. A similar sequencemay be used to form TB2 on FL 14. The present disclosure anticipatesthat the metal (M) in a TB2 metal oxide layer may not be the same metalas in a TB1 metal oxide layer.

In other embodiments, all DMTJ layers 11-17 may be deposited with a PVDprocess in a sputter deposition chamber of a sputter depositionmainframe containing a plurality of deposition chambers and at least oneoxidation chamber. Each PVD step is typically performed in anenvironment comprised of a noble gas such as Ar, and with a chamberpressure that is 5×10⁻⁸ and 5×10⁻⁹ torr. Note that a tunnel barrierlayer such as TB2 15 may be formed by sputter depositing a metal oxidetarget to form a metal oxide layer having a stoichiometric oxidationstate without requiring a separate oxidation step.

FL 14 may be Co, Fe, CoFe, or an alloy thereof with one or both of B andNi, or a multilayer stack comprising a combination of the aforementionedcompositions. In another embodiment, the free layer may have anon-magnetic moment diluting layer such as Ta or Mg inserted between twoCoFe or CoFeB layers that are ferromagnetically coupled. In analternative embodiment, the free layer has a SyAP configuration such asFL1/Ru/FL2 where FL1 and FL2 are two magnetic layers that areantiferromagnetically coupled, or is a laminated stack with inherent PMAdescribed previously with respect to PL1 composition. The FL typicallyhas a thickness between 10 and 30 Angstroms to enhance PMA therein.

Hard mask 17 is non-magnetic and generally comprised of one or moreconductive metals or alloys including but not limited to Ta, Ru, TaN,Ti, TiN, and W. It should be understood that other hard mask materialsincluding MnPt may be selected in order to provide high etch selectivityrelative to underlying DMTJ layers during an etch process that formsDMTJ cells with sidewalls that stop on the substrate 10. Moreover, thehard mask may include an electrically conductive oxide such as RuOx,ReOx, IrOx, MnOx, MoOx, TiOx, or FeOx.

As indicated earlier, we have designed RA₁ for TB1 to be substantiallyless than RA₂ for TB2 in a preferred embodiment of the presentdisclosure. Accordingly, the net DRR for DMTJ 1 b (or 1 c) issubstantially greater than for a DMTJ design where RA₁=RA₂. Moreover,TB1 is below TB2 in the DMTJ stack so that TB1 is a more uniform layerto offset a tendency for a thinner TB1 layer (where t1<t2) to formpinholes that could degrade performance, especially when TB1 has anon-stoichiometric oxidation state where vacant sites in the metal oxidematrix may allow impurities (oxygen or metals) to diffuse across a metaloxide/FL interface.

According to one embodiment shown in FIG. 4, RA₁ in TB1 is engineered tobe substantially lower than RA₂ in TB2 by forming conductive channels 18in a metal oxide (MOx) or metal oxynitride (MON) matrix 13 x. In otherwords, conductive channels formed in a MOx or MON matrix effectivelyrepresent an alternative form for TB1 compared with a substantiallyuniform MOx or MON layer described earlier. The metal M may be one ormore of Mg, Ti, Al, Zn, Al, Zr, Hf, and Ta described previously withregard to TB1 composition. Conductive channels are comprised of a metalor alloy (M2) selected from one or more of Pt, Au, Ag, Mg, Ca, Sr, Ba,Sc, Y, La, Co, Fe, Mn, Ru, Rh, Ir, Ni, Pd, Zn, Cu, Ti, V, Cr, Zr, Nb,Mo, Hf, Ta, Os, and W. Conductive pathways may have a dimension (width)in the in-plane direction that varies from a single atom to a pluralityof atoms. Preferably, each pathway extends from a top surface of PL1 12to a bottom surface of FL 14. Furthermore, the pathways are notnecessarily orthogonal to substrate 10, but may have an in-planecomponent in addition to a substantially vertical orperpendicular-to-plane direction.

Referring to FIG. 5, a method similar to that described for forming adoped metal oxide layer in related patent application Ser. No.15/728,818 may be employed to fabricate conductive channels in aninsulator matrix. According to one embodiment, conductive pathways madeof M2 metal or alloy are formed in a MOx matrix in a reactive gasenvironment generated by a chemical vapor deposition (CVD), physicalvapor deposition (PVD), or a plasma enhanced CVD (PECVD) method whereinthe metal M species, metal or alloy M2 species, and oxygen O species aresimultaneously generated and directed at top surface 12 t of PL1 12, andform a film thereon. The conductive pathways may be formed during theCVD, PVD, or PECVD process, or during a subsequent anneal step thatpromotes M2 diffusion and conglomeration within the MOx matrix. Itshould be understood that the reactive gas environment further includesnitrogen species when a MON matrix is desired. According to the presentdisclosure, the term species comprises one or more of a neutral atom ormolecule, radicals, and positive or negative ions.

According to a second embodiment shown in FIG. 6, a metal M layer 13 mwith top surface 13 t is deposited on PL1 layer 12 during a first step.Then, a second step is performed where the reactive gas environmentmentioned in the previous embodiment is limited to oxygen O species andthe M2 species thereby affording conductive M2 channels 18 in a MO_(x)matrix 13 x, or is limited to N, O, and M2 species to form M2 channelsin a MON matrix in FIG. 4. Again, the conductive channels may be formedduring a CVD, PVD, or PECVD process, or after a film that is a compositeof M, M2 and 0 or a M/M20 film stack (not shown) is annealed to inducediffusion and conglomeration of the M2 atoms into distinct pathways 18.It should be understood that exposure of the metal M layer to 0 and M2species may resputter all or an upper portion of the M layer to generatean intermediate film comprising M, M2, and O, or a bilayer stack with aM/MOM2 configuration, respectively. Thereafter, one or more anneal stepstransform the intermediate film into conductive M2 channels within theMOx matrix (or MON matrix when N species and O species are combined inthe reactive gas environment).

In FIG. 7, a third embodiment for conductive channel formation in aninsulator matrix is provided. First, a MO_(x) (or MON) layer 13 x havingtop surface 13 t 1 is formed on PL1 12. Note that the metal oxide ormetal oxynitride layer may be formed by a sequence involving depositionof one or more M layers followed by an oxidation step such as a naturaloxidation (NOX) process where each of the one or more M layers isexposed to a flow of oxygen (or O₂ and N₂) in a reaction chamber topartially or completely oxidize all metal atoms in the M layer(s).Alternatively, a MO_(x) layer (or MON layer) is deposited with aconventional RF PVD (sputter deposition) process. Thereafter, a reactivegas environment comprised of M2 species is employed to form conductivechannels 18 in the MO_(x) or MON layer. In some embodiments, a M2 layermay be formed on the intermediate MO_(x) or MON layer, and then asubsequent anneal step is used to diffuse the M2 layer into theinsulator layer followed by conglomeration into conductive M2 channels.

Referring to FIG. 8, the present disclosure also anticipates a fourthmethod of forming conductive channels in a MO_(x) or MON matrix. Seedlayer 11 and PL1 12 are sequentially formed on substrate 10. Then afirst MO_(x) or MON layer 13 x 1, a M2 layer 13 m 2, and a second MO_(x)or MON layer 13 x 2 are sequentially formed on the PL1 layer by a PVD orRF PVD process to provide an intermediate trilayer stack. Alternatively,either the first layer 13 x 1 or third layer 13 x 2 may be omitted toyield a bilayer intermediate stack. Thereafter, the remainder of theDMTJ stack including layers 14-17 is deposited on the uppermost layer inthe intermediate bilayer or trilayer stack. The resulting intermediateDMTJ stack of layers in FIG. 8 is transformed into DMTJ 1 b in FIG. 1B(or DMTJ 1 c in FIG. 1C) by performing one or more anneal steps duringfabrication of the memory device. For example, a first anneal step mayoccur before patterning the DMTJ stack into a plurality of DMTJ cells,and a second anneal step may be performed after patterning the DMTJstack of layers. The one or more anneal steps transform the intermediatebilayer or trilayer stack into TB1 having conductive channels in aninsulator matrix.

In all embodiments shown in FIGS. 5-8, a key feature is that the metal Mhas a higher affinity for oxygen than M2 such that M is selectivelyoxidized in the presence of M2. Accordingly, M2 is a metal in Table 1 inFIG. 15 preferably having a less negative free energy of oxide formationvalue than metal M, which is preferably Mg. More preferably, one or moreof the metals in the top half of the table are selected for M2 when M isMg.

Referring to FIG. 9, another embodiment of the present disclosure isdepicted where TB1 in DMTJ 1 b is a doped metal oxide layer 13 d that isa MOD alloy wherein the dopant (D) content is from 100 ppm up to 20atomic %. As we disclosed in related application Ser. No. 15/728,818, adopant that is one of N, S, Se, P, C, Te, As, Sb, or Bi may beintroduced in a metal oxide layer to fill vacant sites in a metal oxidelattice thereby lowering RA. Thus, the dopant will create conductingstates in the band gap of a MgO tunnel barrier layer, for example,through hole generation while providing an additional advantage ofblocking oxygen diffusion hopping through otherwise vacant sites in aTB1 layer with a non-stoichiometric oxidation state.

One of the methods that may be employed to form the doped metal oxidelayer is represented by a process shown in one of FIGS. 5-7 wherein themetal M2 species is replaced by a dopant (D) species. In one embodimentrepresented in FIG. 7 where the M2 species is replaced by a dopantspecies, the dopant species is generated by an ion implantation method.Furthermore, a multistep sequence comprising an intermediate stack shownin FIG. 8 may be used where layer 13 m 2 is comprised of a dopant ratherthan a M2 metal or alloy. Thereafter, one or more anneal steps may beperformed to diffuse the dopant layer into one or both of the metaloxide layers 13 x 1, 13 x 2. In some embodiments, layer 13 m 2 may be aMOD alloy. Preferably, the dopant (D) is one of N, S, Se, P, C, Te, As,Sb, and Bi.

Yet another embodiment of the present disclosure is illustrated in FIG.10. Here TB1 is replaced with a metallic spacer layer 13 m. Although PMAis lowered in FL 14 since there is no longer a second metal oxide/FLinterface to generate additional interfacial perpendicular anisotropy,conductivity is substantially enhanced between PL1 and FL compared witha PL1/TB1/FL stack where TB1 is a metal oxide layer. Moreover, DRR₁ isconsiderably reduced which in turn increases the net DRR in DMTJ becauseRP for the P/AP state and RAP for the AP/P state for the lower p-MTJsubstructure 8 in FIG. 1B or FIG. 1C are proximate to zero therebysubstantially increasing the difference (RA₂−RA₁). However, theimprovement in net DRR and larger difference (RA₂−RA₁) is offset with alarger critical switching current (i_(c)) since PL1 12 effectivelyproduces no spin torque on FL 14 due to the lack of a tunneling effectof electrons between PL1 and the FL.

The present disclosure also encompasses an initialization sequence forforming the magnetic layer orientations shown in FIG. 1B (or FIG. 1C)where there is an AP/P state for DMTJ 1 b (or 1 c) in which PL1magnetization 12 m (or 12 m 1) is antiparallel to PL2 magnetization 16 a(or 16 a 1). Referring to FIG. 11, a first step in the initializationsequence is application of a magnetic field 30 in a vertical (z-axis)direction such that the applied field has sufficient magnitude to setpinned layer magnetizations 12 a, 16 a, as well as FL magnetization 14 ain the same direction as the applied field. In the exemplary embodiment,the applied field 30 is in a (+) z-axis direction. However, in analternative embodiment (not shown), the applied field may be in a (−)z-axis direction to provide magnetizations 12 m, 16 m, and 14 m oppositeto 12 a, 16 a, 14 a, respectively.

According to a second step in the initialization sequence shown in FIG.12, a second applied field 31 is provided in a direction opposite to thedirection of the first applied field, and has a magnitude sufficient toswitch only FL magnetization 14 a to 14 m, and to flip PL1 magnetization12 a to 12 m. As a result, PL1 magnetization is now antiparallel to PL2magnetization 16 a. It should be understood that PL2 16 coercivity mustbe larger than that of PL1 12 coercivity in order to maintain PL2magnetization 16 a during the second applied field.

According to a second initialization process shown in FIG. 13,magnetizations in PL1 12, PL2 16, and FL 14 may be set by application ofan appropriate voltage. A large magnetic field (magnetic field 30) isfirst applied as with the previous method, sufficient to set themagnetizations of PL1, FL and PL2 all parallel to each other. Asdescribed previously in the disclosure, in this configuration (describedin FIG. 1A), the spin torque on the FL from PL1 and PL2 cancel, and arelatively higher write current V_(c,(FL-NWS)) is needed to switch theFL. This aspect of the nonworking state (NWS) is utilized for theinitialization of the stack to the working state (WS). Either PL1 or PL2is designed in such a way that the switching voltage to flip themagnetization V_(c) (_(PL1 or PL2)) is lower than the voltage needed torotate the FL magnetization in the non-working state, i.e. V_(c),(_(FL-NWS))>V_(c) (_(PL1 or PL2))>V_(c) (_(FL-WS)). Once PL1 (or PL2)magnetization rotates, the device goes to the working state where arelatively lower write current V_(c) (_(FL-WS)) also written as Vc,FL_(ws) is required to switch FL magnetization 14 a back tomagnetization 14 m. However, neither magnetization 12 a nor PL2magnetization 16 a switches to magnetization 12 m or magnetization 16 m,respectively, when Vc, FL_(ws) is applied. Thus, Vc, FL_(ws) has thesame effect as l₄ in FIG. 3B.

We have demonstrated the advantage of a DMTJ formed according to anembodiment of the present disclosure by fabricating patterned DMTJ cellswith a critical dimension (CD) varying from 30 nm to 300 nm, and thentesting high speed (10 ns pulse width) switching using a proprietaryshort loop test bed. A first set of DMTJ cells having aPL1/TB1/FL/TB2/PL2 configuration where PL1 and PL2 magnetizations wereinitialized to be parallel (non-working P/P state) was compared with asecond set of DMTJ cells with the same configuration except PL1 and PL2magnetizations were initialized to be antiparallel (working P/AP or AP/Pstate). In each case, write voltages were determined when toggling FLmagnetization such that the non-working state switched back and forthbetween P/P and AP/AP states while the working state switched back andforth between AP/P and P/AP states.

FIG. 14 illustrates the results of the aforementioned experiment.Resistance is plotted vs. voltage where the inner populations 40, 41,represent the V_(c,0) and V_(c,1), respectively, in a working state, andouter populations 50, 51, represent Vc,₀ and Vc,₁, respectively, in anon-working state. Note that V_(c,0) for population 40 corresponds toswitching a P/AP state to an AP/P state, and Vc,₀ for population 50corresponds to switching a P/P state to an AP/AP state. We have found anincrease in the write voltages to a more positive Vc,₀ (40 vs. 50) or toa more negative Vc,₁ (41 vs. 51) for the non-working state compared withthe working state that confirms our original analysis predicting a loweris for the DMTJ 1 b in FIG. 1B (or DMTJ 1 c in FIG. 1C) compared withthe DMTJ 1 a (non-working state) in FIG. 1A.

All of the embodiments described herein may be incorporated in amanufacturing scheme with standard tools and processes. DRR, RA, and isfor a DMTJ are simultaneously optimized by formation of a TB1 in a firstp-MTJ substructure that has a RA₁ substantially less than RA₂ of anoverlying TB2 in a second p-MTJ substructure. Furthermore, PL1magnetization in the first p-MTJ is aligned antiparallel to the PL2magnetization in the second p-MTJ after an appropriate initializationsequence is performed in order to ensure a lower i_(c) compared with ap-MTJ in a single spin filter, or compared with a DMTJ having PL1 andPL2 magnetizations aligned parallel to each other.

While the present disclosure has been particularly shown and describedwith reference to, the preferred embodiment thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of thisdisclosure.

What is claimed is:
 1. A device comprising: a first pinned ferromagneticlayer on a substrate; a metallic spacer formed on the first pinnedferromagnetic layer; a free layer that contacts a top surface of themetallic spacer and having a magnetization aligned orthogonal to thesubstrate; a tunnel barrier layer adjoining a top surface of the freelayer; and a second pinned ferromagnetic layer formed on the tunnelbarrier layer, wherein the second pinned ferromagnetic layer has amagnetization aligned orthogonal to the substrate, and antiparallel to amagnetization of the first pinned ferromagnetic layer.
 2. The device ofclaim 1, wherein the metallic spacer is formed of a material selectedfrom the group consisting of Cu, Cr, Ag, Ge and alloys thereof.
 3. Thedevice of claim 1, wherein the tunnel barrier layer is formed of a metaloxide in which the metal is selected from one or more of Mg, Ti, Al, Zn,Zr, Hf, and Ta.
 4. The device of claim 1, wherein the metallic spacerhas a first resistance x area product, and wherein the tunnel barrierlayer has a second resistance x area product that is greater than thefirst resistance x area product;
 5. A method comprising: forming a firstpinned layer on a substrate; forming a first tunnel barrier layer on thefirst pinned layer, wherein the first tunnel barrier layer has a firstresistance x area product; forming a free layer on the first tunnelbarrier layer; forming a second tunnel barrier layer on the free layer,wherein the second tunnel barrier layer has a second resistance x areaproduct that is greater than the first resistance x area product;forming a second pinned layer on the second tunnel barrier layer; andperforming an initialization process such that a magnetization of thefirst pinned layer is aligned antiparallel to a magnetization of thesecond pinned layer, and wherein a magnetization of the free layer andthe respective magnetizations of the first pinned layer and the secondpinned layer are aligned orthogonal to the substrate.
 6. The method ofclaim 5, wherein the first tunnel barrier layer and the second tunnelbarrier layer include a metal selected from the group consisting of Mg,Ti, Al, Zn, Zr, Hf, and Ta.
 7. The method of claim 5, wherein the firsttunnel barrier layer has a first thickness and the second tunnel barrierlayer has a second thickness, wherein the first thickness is less thanthe second thickness.
 8. The method of claim 5, wherein the first tunnelbarrier layer has a first oxidation state and the second tunnel barrierlayer has a second oxidation state, wherein the first oxidation state islower than the second oxidation state.
 9. The method of claim 5, whereinthe first tunnel barrier layer is formed of a metal oxide material thatincludes a plurality of conductive channels extending therethrough. 10.The method of claim 9, wherein the plurality of conductive channels areinclude material selected from the group consisting of Pt, Au, Ag, Mg,Ca, Sr, Ba, Sc, Y, La, Co, Fe, Mn, Ru, Rh, Ir, Ni, Pd, Zn, Cu, Ti, V,Cr, Zr, Nb, Mo, Hf, Ta, Os, and W.
 11. The method of claim 5, whereinthe first tunnel barrier layer is a doped metal oxide layer thatincludes a dopant selected from the group consisting of N, S, Se, P, C,Te, As, Sb, and Bi.
 12. The method of claim 11, wherein the doped metaloxide layer is formed by a process that includes: forming a metal oxidelayer on the first pinned layer; and exposing the metal oxide layer to areactive environment comprised of a dopant species.
 13. The method ofclaim 11, wherein the doped metal oxide layer is formed by a processthat includes: forming a metal oxide layer on the first pinned layer;and implanting a dopant species into the metal oxide layer.
 14. Themethod of claim 5, wherein the first tunnel barrier layer is formed by aprocess comprising: forming a metal oxide layer on the first pinnedlayer; exposing the metal oxide layer to a reactive environmentcomprised of a second metal species; and performing one or more annealsteps such that a plurality of conductive channels formed of the secondmetal species extend through the metal oxide layer.
 15. The method ofclaim 5 wherein the initialization process includes: applying a firstmagnetic field such that the magnetizations of the first pinned layer,second pinned layer and the free layer are all aligned parallel to eachother in a first direction and orthogonal to the substrate; and applyinga second magnetic field such that the magnetization of the free layerand the magnetization of one of the first pinned layer and the secondpinned layer switches to a second direction while the magnetization ofthe other one of the first pinned layer and the second pinned layerremains oriented in the first direction, the second direction beingopposite the first direction.
 16. The method of claim 5 wherein theinitialization process includes: applying a first magnetic field suchthat the magnetizations of the first pinned layer, the second pinnedlayer and the free layer are all aligned parallel to each other in afirst direction and orthogonal to the substrate; and applying a voltagethat switches the magnetization of the free layer and the magnetizationof one of the first pinned layer and the second pinned layer to a seconddirection that is opposite to the first direction while themagnetization of the other one of the first pinned layer and the secondpinned layer remains oriented in the first direction.
 17. A methodcomprising: forming a first tunnel barrier layer on a substrate, whereinthe first tunnel barrier layer has a first resistance x area product;forming a free layer on the first tunnel barrier layer; and forming asecond tunnel barrier layer on the free layer, wherein the second tunnelbarrier layer has a second resistance x area product that is greaterthan the first resistance x area product.
 18. The method of claim 17,further comprising: forming a first pinned layer on the substrate; andforming a second pinned layer on the second tunnel barrier layer. 19.The method of claim 17, further comprising performing an initializationprocess such that a magnetization of the first pinned layer is alignedantiparallel to a magnetization of the second pinned layer, and whereina magnetization of the free layer and the respective magnetizations ofthe first pinned layer and the second pinned layer are alignedorthogonal to the substrate.
 20. The method of claim 17, wherein thefirst tunnel barrier layer has a first thickness and the second tunnelbarrier layer has a second thickness, wherein the first thickness isless than the second thickness, and wherein the first tunnel barrierlayer has a first oxidation state and the second tunnel barrier layerhas a second oxidation state, wherein the first oxidation state is lowerthan the second oxidation state.